I thought Theresa was more likely someone connected to Sami's trial or imprisonment - a prosecutor or judge (or even a cellmate). The sets from that script indicate courtroom and jail scenes would take place, so it would fit to have a recurring character.

Yes, but the placement of Theresa in the script is with the contract actors and long-time recurring actors (which would include Justin, Adrienne, Victor, etc...) Theresa is alphabetized as though she is a contract character. The other recurring people in that episode, Bev, Rory and Finn all are listed after her name, even though alphabetically they come before Theresa. That's just a hunch I'm going off of, that Theresa is contract, and thus either Jen Lilley or Chrishell Stause.